The present invention relates to an electrode structure for fuel cell which can produce a high current density.
A solid polymer electrolyte fuel cell attracts more attention as an electronic power source of an electric automobile and a space craft compared with a phosphoric acid fuel cell because it is compact and may produce a high current density. In the development of this technical field, various electrode structures, methods of preparing a catalyst and system constructions have been proposed.
A general electrode structure of fuel cell is constructed by integrating, by means of hotpressing, an electrode substance which is prepared by mixing support particles formed by supporting catalyst particles such as platinum on such a carbon support as carbon black with an ionic conductive substance as such Nafion (trademark of Du Pont), with current collectors and an ion exchange membrane to form a sandwich structure consisting of a cathode current collector, a cathode, a solid polymer electrolyte, an anode and an anode current collector.
One of the essential factors required for a fuel cell is to produce a high current density. The density (apparent density) of the carbon support in the catalyst electrode layer of a conventional fuel cell has been over 1.2 g/cm.sup.3. This is because the bulk density of the carbon support employed is high, and the pressure of the hotpressing is also high. When the bulk density of the carbon support in the electrode is high, the volume of cavities in the electrode decreases so that the high current density cannot be disadvantageously obtained. In order to overcome this disadvantage so as to secure sufficient diffusion, the thickness of the electrode has been made thinner, but this method may cause another disadvantage that the absolute amount of the catalyst which participates in an electrode reaction becomes insufficient even if the diffusion can smoothly occur because the supporting amount of the catalyst such as platinum decreases. While the amount of the ionic conductive polymer is required to be increased for elevating the utilization rate of the catalyst, the cavities in the electrode disadvantageously decrease when the addition amount increases.